# Definition for singly-linked list.
class ListNode:
    def __init__(self, x):
        self.val = x
        self.next = None

class Solution:
    def hasCycle(self, head: ListNode) -> bool:
        '''快慢指针'''
        slow,fast=head,head#注意这里和查找中间位置的初始值不同  查找中间位置的fast=head.next
        while fast and fast.next:
            slow=slow.next
            fast=fast.next.next
            if slow==fast:
                return True
        return False